Goal:
    G1 : t  ext : EdgeSet.t  x : VSet.elt  xs : V.t  _b_ : Z  _z_ : VSet.t
  ============================
   (VSet.Equal _z_ _z_)


DEBUG:Going to execute:
clear H
Level 1: Backtrace:

In environment
G1 : t
ext : EdgeSet.t
x : VSet.elt
xs : V.t
_b_ : Z
_z_ : VSet.t
The term "Z" has type "Set" while it is expected to have type 
"Prop" (universe inconsistency: Cannot enforce Set <= Prop).